The present invention relates to a method for reducing the resistance of a perovskite-type transition-metal oxide SrTiO3.
A perovskite-type transition-metal oxide SrTiO3 has a wide bandgap energy of 3.3 eV. Therefore, SrTiO3 gets attention as a base material of other perovskite-type transition-metal oxides, a nonlinear resistive element, a ferroelectric material or the like, and a SrTiO3 film having a high dielectric constant are actively developing.
A high dielectric film consisting of SrTiO3 is typically prepared through a spattering method using a SrTiO3 sintered body as a target (e.g. Japanese Patent Laid-Open Publication Nos. 6-140385 and 9-153598).
On the other hand, it has been difficult to prepare a p-type SrTiO3 having a desirably lowered resistance even by adopting any conventional technologies. Specifically, a conventional method of doping a p-type dopant N (nitrogen) by itself has been able to increase carrier concentration only up to about 1014 cmxe2x88x923 due to the deep impurity level and resulting low activation rate of N acceptor. Further, the doping of N as an acceptor causes a compensation effect, which inevitably leads to oxygen deficiency serving as a donor. Such negative factors obstruct to achieve a desired low-resistance p-type SrTiO3.
The inventors have found a new technical concept of simultaneously doping or co-doping an acceptor with a donor having a high chemical bonding strength with the acceptor intentionally to provide a shallowed accepter level and suppressed compensation effect, and have verified that the above problem could be solved by the developed technique.
According to a first aspect of the present invention, there is provided a method for preparing a low-resistance p-type SrTiO3, wherein an acceptor and a donor are co-doped into a perovskite-type transition-metal oxide SrTiO3 during crystal growth.
According to a second aspect of the present invention, there is provided a method for preparing a low-resistance p-type SrTiO3, wherein an acceptor (A) and a donor (D) are co-doped into a perovskite-type transition-metal oxide SrTiO3 during crystal growth to form an impurity complex (A-D-A) of the acceptor and the donor through a crystal growth method in a thermal nonequilibrium state, whereby an impurity formation energy is reduced by less than that in a method of doping the acceptor by itself, so as to provide an increased concentration of the acceptor.
According to a third aspect of the present invention, there is provided a method for preparing a low-resistance p-type SrTiO3, wherein an acceptor (A) and a donor (D) are co-doped into a perovskite-type transition-metal oxide SrTiO3 during crystal growth to form an impurity complex (A-D-A) of the acceptor and the donor through a crystal growth method in a thermal nonequilibrium state, whereby a Coulomb scattering mechanism having a long range force is converted into a dipole or multipole scattering mechanism having a short range force, so as to provide an increased mobility of the impurities.
According to a fourth aspect of the present invention, there is provided a method for preparing a low-resistance p-type SrTiO3, wherein an acceptor (A) and a donor (D) are co-doped into a perovskite-type transition-metal oxide SrTiO3 during crystal growth to form an impurity complex (A-D-A) of the acceptor and the donor through a crystal growth method in a thermal nonequilibrium state, whereby the acceptor has a shallower impurity level than that in a method of doping N by itself, so as to provide an increased activation rate of the acceptor.
In the present invention, in order to prevent oxygen deficiency, the donor may be Nb oxide, and the acceptor may be N oxide.
Further, the donor may be an n-type dopant of Nb (niobium), and the acceptor may be a p-type dopant of N (nitrogen). In this case, the n-type dopant of Nb and the p-type dopant of N are co-doped at a rate of 1:(1+x) where 0 less than x less than 100, so as to provide p-type carriers doped in a high concentration of 1019 cmxe2x88x923 to 1021 cmxe2x88x923. In the ratio of Nb (niobium) to N (nitrogen), the amount of N should be greater than that of Nb to obtain a p-type SrTiO3, and a significant co-doping effect can be observed at the ratio of up to about 1:100.
In the method of the present invention, the SrTiO3 crystal can be grown on a substrate such as Si, SrTiO3 or xcex2Al2O3 by employing an MBE (Molecular Beam Epitaxy) method, laser-MBE method, CVD (Chemical Vapor Deposition) method or spattering method.
The CVD method promotes crystal growth under hydrogen atmosphere, and a hydrogen donor will remain in the resulting crystal because hydrogen acts as a donor. However, the remaining donor can be removed by subjecting the resulting crystal to annealing (at about 500xc2x0 C.), and thereby an acceptor acts to provide a desirable low-resistance p-type crystal.